# Functions / Transformations

### Overview

Transformation functions in JENTIS allow you to process and modify data before it is sent to a destination (e.g., analytics tools, marketing platforms). These functions take a **single input** and return a **transformed output** — both of type `String`.

### How to Create a Custom Function

Creating a custom transformation is simple. All you need is a **JavaScript function** that:

* Accepts **one input parameter** of type `String`
* Returns an **output value** of type `String`

#### Example:

```javascript
function myTransformation(input) {
  return input.toLowerCase();
}
```

This function would take a string (e.g., `"MyValue"`) and return it in lowercase (`"myvalue"`).

### Common Use Cases

Transformation functions can be used for a wide range of purposes, such as:

* Converting strings to lowercase
* Removing all whitespace from a string
* Filtering out disallowed URL parameters
* Matching and transforming values based on a specific syntax or pattern

These transformations are especially useful for data cleaning, formatting, or standardization before sending the data to third-party tools.

### Built-In Functions: Anonymization & Pseudonymization

The JENTIS system provides built-in support for **Anonymization** and **Pseudonymization** functions, which can be directly applied in the **Tag Edit View**.

* [**Anonymization**](https://docs.jentis.com/developer-guide/functions-transformations/anonymization-and-pseudonymization)**:** Generates a random output each time, preventing traceability.
* [**Pseudonymization**](https://docs.jentis.com/developer-guide/functions-transformations/anonymization-and-pseudonymization)**:** Generates a consistent but obfuscated output per tool, ensuring format similarity while preserving privacy.

To learn more about these privacy-focused transformations, see the [#built-in-functions-anonymization-and-pseudonymization](#built-in-functions-anonymization-and-pseudonymization "mention") Guide.